1. Field of the Invention
The present invention relates to a communication terminal device for transmitting and receiving frames in a communication network system, and more particularly to a communication terminal device for transmitting and receiving frames each containing a plurality of packets.
2. Description of the Background Art
In a conventional communication network system where frames are transmitted and received between a plurality of communication terminal devices, data transmission is conducted by transmitting a frame, which contains a set of header information, data, and frame check sequence information, from a communication terminal device to a reception terminal device (see pages 34 and 199-205 of ANSI/IEEE Std802.11, ISO/IEC 8802-11:1999(E)).
FIG. 9 is a diagram showing a conventional frame structure disclosed in ANSI/IEEE Std802.11, ISO/IEC 8802-11:1999(E). In FIG. 9, a frame includes a preamble field 91, a frame control field 92, a duration/ID field 93, a first address field 94, a second address field 95, a third address field 96, a frame body field 97, and a frame check sequence field 98.
The preamble field 91 contains information used for establishing frame synchronization. The frame control field 92 contains a frame type, etc. The duration/ID field 93 contains information related to a length of time for which a communication medium is occupied for transmitting the frame. The first address field 94 contains a destination terminal device address indicating an address of a terminal device, which receives the frame, among terminal devices which directly transmit/receive the frame. The second address field 95 contains a source terminal device address indicating an address of a terminal device, which transmits the frame, among the terminal devices which directly transmit/receive the frame. The third address field 96 contains a source address indicating an address of a terminal device from which the frame was originally transmitted, and a destination address indicating an address of a terminal device which is a final destination of the frame. The frame body field 97 contains a sequence number, cryptographic information, and data. The frame check sequence 98 contains information used for detecting whether the frame has an error.
In the frame shown in FIG. 9, the preamble field 91, the frame control field 92, and the duration/ID field 93 are transmitted at a lowest possible speed using a modulation technique, which provides high redundancy, so as to be recognized by all terminal devices on the communication network. Other fields (hereinafter, correctively referred to as a “data field”) are transmitted at high speed using a modulation technique suitable for a communication status between communicating terminal devices.
As described above, the preamble field 91, the frame control field 92, and the duration/ID field 93 are transmitted at a lowest possible transmission rate, while the data field is transmitted at a high transmission rate. However, if the transmission rate of the data field is considerably high, overhead due to transmission of the preamble field 91, the frame control field 92, and the duration/Id field 93 is increased. Accordingly, an effective transmission rate is reduced.
For example, consider a case where a working bandwidth and the data transmission rate are quadrupled in a system which conforms to the IEEE802.11a standard and uses the 5 GHz band. It is assumed that a 1500 byte data field is transmitted as an Ethernet® packet. In this case, it requires 24 microseconds (μsec) to transmit the preamble field 91, the frame control field 92, and the duration/ID field 93. Further, it requires 56 μsec to transmit the data field. Accordingly, 56 μsec/(24 μsec+56 μsec)=0.7, and therefore a transmission efficiency for the data field is reduced to 70%. Moreover, in the case of using an access control system, such as CSMA/CD, it is necessary to insert a gap between frames to be transmitted, so that the transmission efficiency is reduced to 40% or lower on average.
In order to solve the problem as described above, Japanese Patent Laid-Open Publication No. 7-123118 proposes a frame transmission circuit. This frame transmission circuit transmits transmission frames each containing a plurality of packets, thereby increasing the transmission efficiency.
The frame transmission circuit proposed by Japanese Patent Laid-Open Publication No. 7-123118 is applicable to a case of transmitting only data packets. However, this frame transmission circuit is not able to satisfy quality of service (QoS) requirements, e.g., a maximum delay time, jitter prevention, etc., required by an AV stream, such as an audio packet (e.g., a VoIP packet) or a video packet (e.g., an MPEG packet), to be satisfied, and therefore is not applicable to a case of transmitting the AV stream.
Further, in the case of using radio or a power line as a communication medium, a frame error rate is increased from 0.1% to approximately 1.0%, and therefore if a terminal device on the reception side detects an error using forward error correction (FEC), a terminal device on the transmission side might be frequently required to retransmit frames. However, retransmission in units of frames impairs a retransmission efficiency, and therefore it is not possible to increase throughput. In order to solve this problem, retransmission in units of packets is conceivable. However, if the frame contains variable-length packets, it is difficult to specify the location of a packet, and therefore it is difficult to detect an error for each packet.